Solution on Windows 2003
1. Open Windows Explorer, locate the folder with your website, right-click the folder and display its Properties. Choose the Security tab and click Add....
2. The Select Users, Computers and Groups dialog appears. Click Locations....
3. Choose your local computer and click OK.
4. Enter network service into the box and click Check Names. The name should be resolved to NETWORK SERVICE. Click OK.
5. The account is added to the list of accounts. Grant the Modify permission to the account and click OK.